One of the great things in Stronghold are the portable powerups that are rarely found in maps but always available in the shops of the Mission Control (INTERMAP/MAP99). Some of them are very powerful and can be only bought in the later stages of the game, so be sure that you might spoil the surprise if you read this thread now!

Marine Reinforcements
Using this powerup spawns a Marine Reinforcement unit with a random weapon directly in front of you, fighting with you against the hordes of monsters. Especially in the early mission stages, these cheap support units are a great help. Mostly they come equipped with standard weapons like shotguns or chainguns, but if you have a lucky hand, they will even come with a plasma gun.

Max. carriable amount: 10

Shop price: x $

Random Weapon Generator
This powerup gives you one random but powerful weapon of any type at the moment you use this. This can definitely change the outcome of a whole defense session considering that you already might get a ssg or even a plasmagun or bfg from the first wave on. It's a game of luck, but definitely worth the money.

Max. carriable amount: 3

Shop price: x $

Portable Defence Barrier
Use this to place a high energetic wall in front of you to block some parts of the map for a while. It's vulnerable and doesn't last forever but you can block off monsters and also attacks for a while with this barrier.

Max. carriable amount: 3

Shop price: x $

Smart Bomb
The smart bomb is a very powerful weapon and can save your life in hot situations. Throw it, get yourself in a secure place and watch the bomb do the rest. Every single demon in sight gets killed or at least receives heavy damage. Be careful though: It also kills you, your reinforcements and your comerades.

Max. carriable amount: 1

Shop price: x $

Sentry Turret Unit
This powerup places a sentinel drone directly in front of you. A sentinel drone is a static plasma gun that automatically fires at incoming units. It's quite powerful and its bullets only damage enemy creatures, not you and not your comerades. Beyond, it takes a lot of damage before it is destroyed. Place it in strategic smart positions to gain some time.

Max. carriable amount: 3

Shop price: x $

Regeneration Sphere
This powerup regenerates life while it's lasting. The best way to use it is while you are battling demons and you have no time to check back for health backup, as this thing automatically does the job. It's not much, and it's not fast, but it might be a good backup in certain situations.

Max. carriable amount: 3

Shop price: x $

Protection Sphere
The protection powerup simply does what it's called like: It protects you from heavy damage by cutting the overall damage down to only a small percentage. Works best in melee combat or against bosses that deal lots of damage, as long as the protection sphere lasts.

Max. carriable amount: 3

Shop price: x $

Weapon Enhancements
This little nifty powerup enhances all of your weapons in a technical way to fire faster, produce more damage or totally change the weapons behavior. It's very powerful and with its help, you can nullify a huge horde of monsters within seconds. (it's actually the equivalent to the "Tome of Power")

Max. carriable amount: 3

Shop price: x $

Morphsphere
It's actually a morphing powerup that turns you into a Bruiser Demon, oh yeah, double the fun. Toss fireballs at your enemies and blast them with the Bruiser's powerful ground attack, it's all yours now. Only lasts for a short while but it works.

Max. carriable amount: 3

Shop price: x $

I-Win-Button
The most powerful powerup of all: The "I Win Button" actually combines several mighty powerups into one, just try it, you will love it and its worth every single credit of its price!

Max. carriable amount: 1

Shop price: x $

Invulnerability Sphere
It's simply a portable form of the invulnerability sphere, nothing else.

Max. carriable amount: 3

Shop price: x $

Life (1-Up)
It's pretty expensive to get your guts and bones from the battlefield and resurrect your poor soul in the labs of the UAC, but it's achievable. With this item you can get back to the battlefield for yet another time.

Max. carriable amount: 9

Shop price: x $

Time Freezer
If you're in a hurry and need some time to kill demons or back yourself up, then simply use the Timefreezer powerup to gain some more time while others have to wait. Very useful in situations where your enemies already got your balls tight.

Max. carriable amount: 3

Shop price: x $

Auto Drone
The more intelligent and more powerful variant of a Sentinel Turret is the actual Drone. It has a high firepower, can't be destroyed, follows you and fires at everything that's moving and not looking like a marine. It doesn't last long but it's worth the money. The cool thing about it is: you can use two of them at the same time!

Max. carriable amount: 2

Shop price: x $

Quad Damage Amplifier
If there's one thing in Quake that deserves being mentioned, then it is Quad Damage. 4 times the firepower for a short moment of time makes you mow down everything in sight if you have enough ammo.

Max. carriable amount: 3

Shop price: x $

Portable Barrel
Oh yea, the good ol' barrel. If you need to stop demons for a little while or if you need additional hazardous things standing around here and there, place a barrel or two.

Max. carriable amount: 10

Shop price: x $

Vampirism Sphere
Drains the health of your opponents and gives it back to you. Use it if you have enough ammo and a powerful weapon at your hands and you will never need to get a medikit again... until the effect is over of course!

To expand upon what Torm said, you need to use the key that has been bound to your "use inventory" function. Open your game and go into your "Options". Select "Customize Controls" and scroll down to the "Inventory" section. The first selection is "Activate Item" - this is the "use" key, and if there is no default key binding (or you want to change it) you can put in your own. This is what you'll use to activate an item in your inventory. You'll use the '[' and ']' keys (the typical default bindings) to scroll through your inventory.